Conventional synthesis gas generating processes include steam reforming, gas phase partial oxidation and autothermal reforming. Each of these processes has advantages and disadvantages when compared to each other.
In a steam reforming process, steam is reacted with a hydrocarbon containing feed to produce a hydrogen-rich synthesis gas. The general stoichiometry, as illustrated for methane, is:CH4+H2O→CO+3H2  (1)Typically, an excess of steam is used to drive the equilibrium to the right. As applied to hydrogen manufacture, excess steam also serves to increase water gas shift:CO+H2O→CO2+H2  (2)
Because of the high endothermicity of the reaction, steam reforming is typically carried out in large furnaces, in which the catalyst is packed into tubes. The tubes must withstand the high pressure of the produced synthesis gas, while transmitting heat at temperatures approaching 1000° C. As described in Stanford Research Institute International Report No. 148A (1995), steam reforming process efficiency, defined as the heat of combustion of product synthesis gas divided by the heat of combustion of reforming feed and furnace fuel, is approximately 79%, while the space velocity, defined as Standard Cubic Feet per Hour of C1-equivalent feed/ft3 of catalyst bed is 690 hr−1. Unfortunately, steam reforming furnaces occupy a very large volume of space, orders of magnitude greater than the tube volume, such that low productivity limits the economic attractiveness of the process.
The gas phase partial oxidation process involves the partial oxidation of the hydrocarbon containing feed in the gas phase. The feed components are introduced at a burner where they combust with sub-stoichiometric oxygen to produce a synthesis gas mixture. The ideal gas phase partial oxidation reaction, as illustrated for methane, is:CH4+½O2→CO+2H2  (3)However, gas-phase reaction kinetics tend to over-oxidize some of the feed, resulting in excessive heat generation and substantial yield of H2O, CO2, and unreacted hydrocarbons that my leave the reactor as soot.
For these reasons, when gas phase partial oxidation chemistry is applied to clean feeds, it is preferred to add steam to the feed and add a bed of steam reforming catalyst to the bottom of the gas phase partial oxidation reactor vessel. This combination of gas phase partial oxidation and steam reforming is called autothermal reforming. The heat needed for steam reforming is provided in-situ from the excess heat generated by the gas phase partial oxidation reactions.
Autothermal reforming can be substantially more compact than steam reforming because furnaces are not used to provide heat of reaction. Embodiments of autothermal reforming presently under development for gasoline-powered fuel cell vehicles make use of a catalytic partial oxidation step, which can impart even greater compactness to the autothermal reforming process. The thermal efficiency of autothermal reforming reactors are generally in the range of 90%. While autothermal reforming based on catalytic partial oxidation may have high productivity (gas hourly space velocity on order of 104 as C1-equiv.), conventional partial oxidation-based autothermal reforming has a space velocity very similar to that of steam reforming (about 103 hr−1). However, autothermal reforming requires a source of oxygen. In the fuel cell vehicle case, this oxygen in typically provided as low-pressure air, which results in a nitrogen-diluted, low-pressure synthesis gas, while in refinery or chemicals embodiments, this oxygen is typically provided as purified O2, and the cost of air separation can be greater than the cost of the autothermal reforming process.
Sederquist (U.S. Pat. Nos. 4,200,682, 4,240,805, 4,293,315, 4,642,272 and 4,816,353) teaches a steam reforming process in which the heat of reforming is provided within the bed by cycling between combustion and reforming stages of a cycle. As described by Sederquist, the high quality of heat recovery within the reforming bed results in a theoretical efficiency of about 97%. However, the examples and commercial projections within these patents describe a process that operates at very low productivity, with space velocities of around 95 hr−1 (as C1-equiv). Moreover, this process requires a compressor to compress the product synthesis gas to useful pressures for hydrocarbon synthesis.
Recently a highly efficient and highly productive process for producing synthesis gas in a cyclic, packed-bed operation has been discovered. In this process, the reforming step involves preheating a first zone to a temperature in the range of about 700° C. to 2000° C. and then introducing a 20° C. to 600° C. hydrocarbon-containing feed, along with steam and optionally CO2 to the inlet of the first zone. Upon introduction of the reactants, the hydrocarbon is reformed into synthesis gas over a catalyst in this first zone. The synthesis gas is then passed from the first zone to a second zone, where the gas is cooled to a temperature close to the inlet temperature of the hydrocarbon feed. The synthesis gas is recovered as it exits the inlet of the second zone.
The regeneration step begins when a gas is introduced to the inlet of the second zone. This gas is heated by the stored heat of the second zone to the high temperature of the zone and carries the heat back into the first zone. Finally, an oxygen-containing gas and fuel are combusted near the interface of the two zones, producing a hot flue gas that travels across the first zone, thus re-heating that zone to a temperature high enough to reform the feed. Once heat regeneration is completed, the cycle is completed and reforming begins again.
An advantage of this process is the ability to operate the reforming step at a higher pressure than the regeneration step, thus creating a pressure swing, and producing high pressure synthesis gas.
Processes for producing liquid and gaseous hydrocarbon products from synthesis gas are known. These include Fischer-Tropsch synthesis, methanol synthesis and hydroformylation. Each of these are exothermic processes that operate best at a H2:CO molar ratio of about 2.
Fischer-Tropsch synthesis typically is carried out using a cobalt or iron catalyst at temperatures of between 200° C. to 450° C. and pressures of between 10 and 300 atmospheres. The hydrocarbon product is roughly equivalent to a very paraffinic, natural petroleum oil that contains predominantly straight chain, saturated paraffins, some olefins and less than about 1% alcohols, fatty acids and other oxygenates.
Methanol synthesis typically is carried out using copper-zinc oxide-alumina catalysts at pressures of about 50–100 atm. and temperatures of about 200° C.–300° C. The by-products of methanol synthesis includes ethers, formates, ketones, hydrocarbons, and higher alcohols.
Hydroformylation involves the reaction of olefins with CO and H2 and typically is carried out in the liquid phase in the presence of a metal carbonyl catalyst at temperatures in the range of 50° C. to 200° C. at pressures of 10 to 200 atm.
The practical application of any synthesis gas production technique or hydrocarbon conversion process will depend upon how well the upstream and downstream processing systems can be integrated into an overall process design. The invention described below and defined in the claims addresses practical process design and operating requirements that achieve effective integration of pressure swing reforming with downstream hydrocarbon conversion and that provides unanticipated advantages.